Superantigens stimulate a large proportion of T cells through a specific interaction with the beta-chain of the T-cell receptor (TCR). Thus, an individual superantigen such as Mls-1 or staphylococcal enteroxin A, when presented by MHC class II, stimulates T cells bearing specific Vbeta-regions. This project aims to analyze the affinity and structure of the interaction between the TCR beta-chain and the complex of MHC class II with bacterial or retroviral superantigens. A secreted form of the TCR beta-chain and mutant staphylococcal enteroxin superantigens have been used to demonstrate the Vbeta binding site of the superantigen. Recombinant retroviral superantigens will be prepared for studies on the interaction with MHC and TCR. The affinity and rate constants for the reaction of various V-betas with superantigens will be determined using surface plasmon resonance. This will allow a minimal estimate of the affinity required for a beta chain to undergo thymic deletion or peripheral activation. This information will be extremely important in understanding the basis of tolerance induction by negative selection of the thymus. Bacteriophage-display systems will be used to produce new superantigens with different properties. They will be selected for binding to class II or TCR on cells or in solid phase. A major aim is to produce a mutant SEA with high affinity binding for TCR so that it should cause anergy in T cells. It will also be useful for co-crystallization of the TCR with the superantigen.